This invention relates to power input controls for motors, and particularly to a power-input control for a three-phase AC induction motor which provides power proportional to loading on the motor.
So far as prior art is concerned, this invention is related to the invention described in U.S. Pat. No. 4,052,648 to Nola. The Nola patent describes a control system for AC single-phase induction motors which monitors the power factor (cos of the phase difference between the voltage signal applied to the motor's winding and current drawn by the winding) and utilizes the phase difference to inversely vary the duration of "on" time of each cycle of input power to the motor. Thus, when the motor's load is decreased, and its phase angle increases, the system described in the Nola patent reduces the "on" time of each cycle of input power to the motor to save energy. The information contained in the Nola patent is hereby incorporated herein by reference. In Column 4, beginning at line 62 of the Nola patent, Nola suggests that his invention could be used with a three-phase induction motor by using three of his control systems, each being connected in one of the three phase windings. In fact, such arrangements have proven to be impractical in that they require multiplicity of expensive parts and do not work efficiently, and with stability, because of a lack of proper isolation between certain circuit subsystems. Thus, it is an object of this invention to provide a power-factor control system for induction motors that has proper isolation between circuit subsystems and which works effectively, and with stability.
Another problem with three-phase, power-factor control systems for induction motors suggested in the prior art is that they must be coupled to the neutrals of wye connected induction motors, which are sometimes difficult to reach. Thus, it is an object of this invention to provide a three-phase, power-factor control system for induction motors which can be utilized to effectively control the power consumption of motors without gaining access to the motors' neutral terminals.
It is also an object of this invention to provide an effective power-factor actuated control system for multi-phase AC induction motors.
It is also an object of this invention to provide a power-factor control system for multi-phase motors which is accurate in operation and practical to build and install.